"There are a number of possibilities for what happened to the ship, right from it having slowed down because of bad weather and it could arrive tomorrow, or to it having sunk, but the facts don't point to either of those scenarios," he said.

"So there are two fairly strong lines of possibility that people are looking at. The first is that the pirates remain on board and the ship was in fact hijacked and that when they got to the Straits of Gibraltar, they continued south down the west coast of Africa in order to sell the cargo or the ship.

"The second possibility is that the crew is complicit, is that the crew have effectively stolen the ship."

Mr Gibbon-Brooks says the case has caught authorities off guard.

"There are lots of instances of cargoes going missing and cargo crime and fraud," he said.

"I think what is unusual about this and what's got world attention is the fact that potentially, if the attack occurred as advertised, there's been a piratical attack in Swedish territorial waters.

"I think probably the one question we ought to put to bed is; are there gangs of Swedish pirates floating about looking to attack the next ship that passes through the Baltic? That seems fairly unlikely. It's more likely that the ship was targeted specifically."

The fact there has been no ransom request makes him believe a "commercial dispute" may be to blame.

"There's no demands that have been made, and I think this may well be a dispute that's been on-running for maybe a couple of years, and that they've seen how easy it is and how well the Somalis can do it," he said.

"So people have basically used that blueprint for their own means to recover the vessel. Or it could be an insurance job.

"This is very, very unique. This is an oddball, and it is an actual act of piracy, and it's the first time it's happened in European waters for a few hundred years."

He believes the ship will be repainted, renamed and taken off the radar permanently.